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Healing-Phase, Top-Fill Traps Associated With Mass Transport Complexes and Components: An Under Explored Opportunity in Deepwater Systems


Mass transport processes and deposits make up a large component of deepwater basin fills around the world. Although their role as reservoir rock is limited, they do play multiple important roles in the formation and fill of deepwater basins, including, but not limited to, 1. erosion and destruction of older reservoirs, 2. deformation of younger reservoirs and seals through compaction and upward fluid ejection, 3. creation of extensive, sometimes overpressured, seals for adjacent or underlying reservoirs, and 4. creation of accommodation for healing-phase gravity flow sediment accumulations. These later deposits, here termed healing-phase, top-fill traps (HPTT) can form significant reservoir volumes in traps whose basal and lateral seals are formed by the underlying mass transport material and whose topseal is formed by the overlying pelagic shales which drape these deposits. Accommodation and fill nature vary within the three deformational domains of the MTC; the proximal extensional, middle translational and distal compressional domains. The extensional domain shows strike elongate distribution of accommodation, and the deposition of HPTT fills with a high probability of sand, and a lower risk of charge with extensional faults intersecting the decollement and associated underlying source rocks. The HPTT within the compressional domain are contained in similar strike elongate accommodation sinks. In addition, thrust faults here also rooted in the decollement provide charge pathways, but the distal nature of compressional-domain accommodation, presents a higher risk to reservoir quality. Translational domains provide the most expansive zone of underfilled accommodation to host post-movement turbidites, however the HPTT basal surface rugosity can present several problems in production. Compaction of HPTT fill can increase dewatering and the risk of topseal breach. In addition, HPTT fills within translational regions lack the decollement-attached faulting to charge HPTT reservoirs, presenting an increased risk of migration into these regions. Some examples of successful exploration in these settings include the North Slope Alaska Cretaceous and in some areas of the North Sea. However, the extensive nature of mass transport deposits in basins throughout the world suggest much potential exists.